[clang][Interp] Fix checking null pointers for initialization

Author: tbaederr

clang/lib/AST/Interp/EvaluationResult.cpp

@@ -141,7 +141,9 @@ bool EvaluationResult::checkFullyInitialized(InterpState &S,
                                              const Pointer &Ptr) const {
   assert(Source);
   assert(empty());
-  assert(!Ptr.isZero());
+
+  if (Ptr.isZero())
+    return true;
 
   SourceLocation InitLoc;
   if (const auto *D = Source.dyn_cast<const Decl *>())

clang/test/AST/Interp/cxx20.cpp

@@ -814,3 +814,16 @@ namespace GH64949 {
                               // both-note {{subobject 'g' is not initialized}} \
                               // both-warning {{expression result unused}}
 }
+
+/// This used to cause an assertion failure inside EvaluationResult::checkFullyInitialized.
+namespace CheckingNullPtrForInitialization {
+  struct X {
+    consteval operator const char *() const {
+      return nullptr;
+    }
+  };
+  const char *f() {
+    constexpr X x;
+    return x;
+  }
+}